This invention relates in general to an electrophotographic imaging process, and more specifically to a method for improving the cleanability of electophotographic imaging surfaces.
The formation and development of images on the surface of photoconductive materials by electrostatic means is well known. The basic xerographic process as taught by C. F. Carlson in U.S. Pat. No. 2,297,691, involves placing a uniform electrostatic charge on a photoconductive insulating layer, exposing the layer to a light and shadow image to dissipate the charge on the areas of the layer exposed to the light and developing the resulting latent electrostatic image by depositing on the image a finely divided electroscopic material referred to in the art as "toner." The toner will normally be attracted to those areas of the layer which retain a charge, thereby forming a toner image corresponding to the latent electroscopic image. This powder image may then be transferred to a support surface such as paper. The transferred image may subsequently be permanently affixed to the support surface by heat. Other suitable fixing means such as solvent or overcoating treatment may be substituted for the foregoing heat fixing step.
The methods which have been employed to develop images in electrophotographic printing processes are many and varied. They include cascade development, powder cloud development, magnetic brush development and other methods including fur brush development, doner belt development, impression development and liquid spray development. The two methods most frequently employed in commercial office copying machines which make use of reusable electrophotographic insulators are cascade development and magnetic brush development. The toner particles applied to these development processes usually consist of one or more thermoplastic resin binder materials, for example, polystyrene, polymethyl styrene, polymethyl methacrylate, styrene methacrylate copolymers, and like materials, mixed with from about 1-20% by weight of a coloring material such as carbon black or a colored pigment, so that a colored image can be easily heat fused onto a copy sheet.
The transfer of the toner to the paper is by electricial attraction. Electrical transfer is accomplished by placing the paper in contact with the imaged area of the photoconductive insulating layer, charging the paper electrically with the same polarity as that of the latent image, and then stripping the paper from the plate. The charge applied to the paper overcomes the attraction of the latent image for the toner particles and pulls them onto the paper. Another technique for electrostatic transfer utilizes a semiconductive roll. A dc potential of the correct sign and voltage is applied between the roll and the electrode of the reusable electrophotoconductive insulating layer.
Complete transfer of toner from the surface of the reusable photoconductive insulating layer to the paper is not accomplished by these transfer methods. Accordingly, a fraction of the toner remains behind on the surface of the resusable electrophotoconductive insulating layer, and this residual toner must be removed prior to the next imaging cycle using a suitable cleaning device.
Various electrostatic plate cleaning devices such as "brush" cleaning apparatus, the "web" type cleaning apparatus and the "blade" cleaning apparatus are known in the prior art. A typical brush cleaning apparatus is disclosed by L. E. Walkup et al. in U.S. pat. No. 2,832,977. Brush type cleaning means usually comprise one or more rotating brushes, which brush residual powder from the plate into a stream of air which is exhausted through a filering system. A typical web cleaning device is disclosed by W. P. Graff, Jr. et al. in U.S. Pat. No. 3,186,838. As disclosed by Graff, Jr. et al., removal of the residual powder from the plate is effected by passing a web fibrous material over the plate surface. Blade cleaning involves contacting the plate surface with a flexible cleaning blade which wipes residual toner off the surface of the plate. Typical blade cleaning techniques are disclosed in U.S. Pat. Nos. 3,552,850 and 3,635,704.
The sensitivity of the imaging member to abrasion, however, requires that special precautions be exercised during the cleaning phase of the copying cycle. For example, pressure contact between cleaning webs or blades and imaging surfaces must be kept to a minimum to prevent rapid destruction of the imaging surface. Although thick protective coatings would protect the imaging surfaces for longer periods of time, the electrical properties of the photoconductive layer impose certain limitations as to the acceptable maximum thickness of the coating. Since thick protective coatings are normally applied by conventional coating techniques, including the use of a film forming material suspended in a solvent, considerable inconvenience, expense and time is involved in removing the photoreceptor from the machine, preparing the eroded photoreceptor surface for reception of a new coating, applying the new coating, allowing the new coating to dry and reinstalling the newly coated photoreceptor into the machine. Certain extremely thin films, applied to the imaging surface as a pretreatment or in situ during the machine sequence, have been successful; however, the art is constantly on the lookout for improved films or at least practical alternatives. Further, for reasons which are not entirely clear, toner particles are frequently difficult to remove from some photoreceptor coating materials, and toner accumulation causes deterioration of subsequent images formed on the photoreceptor surface in reusable imaging systems. Thus, there is a continuing need for a better system for protecting imaging surfaces, developing electrostatic latent images and removing residual development images.
The most effective approach in overcoming the aforementioned problems has been to incorporate a minor amount of an additive material into the toner or developer mixture used in the electrophotographic process. Some additives facilitate the cleaning of the plate surface by reducing the adhesion of the toner to the plate surface. For example, toner filming is reduced according to the disclosure of British Pat. No. 1,233,869 by using as an electrophotographic developer a composition containing minor amounts of polyethylene or certain fluorine containing polymers. Other additives facilitate cleaning by reducing the frictional forces between the plate surface and a cleaning member. Examples of such additives are fatty acids or fatty acids salts such as are disclosed in U.S. Pat. No. 3,552,850. However, the mere fact that a particular material has known lubricating properties, or is of low free surface energy, does not necessarily mean that it will be effective as a plate cleaning additive. Other characteristics such as effect on triboelectric properties of the developer, tendency to cause agglomeration of the developer, resistance to abrasion, and most significantly, the effect of the additive on image quality, come into play in determining whether or not a particular material has utility in an electrophotographic process.
Accordingly, it is an object of this invention to provide an electrophotographic process whereby toner film formation on photoreceptor surfaces is reduced.
Another object of this invention is to provide a method whereby the frictional forces in an electrophotographic imaging process are reduced between the photoreceptor surface and the cleaning member employed to remove residual toner from said surface.